Highly functionalized, polar perfluorinated polyethers (“PFPE”) are currently used as lubricants in a variety of high-performance applications, for example to lubricate hard disc drive (“HDD”) media, as well as to lubricate precision mechanical instruments to minimize mechanical wear. PFPEs also function as anti-wetting and/or corrosion-protective agents on metal-containing substrates, particularly in magnetic recording devices such as magnetic recording disks and magnetic recording heads. See, for example, U.S. Pat. No. 6,638,622 to Dai et al., which describes novel metal salts of perfluorinated polyethers.
One important area in which perfluorinated polyethers find utility is as lubricant layers in magnetic storage devices such as magnetic storage disks and magnetic recording heads. Of particular interest are magnetic storage disks and magnetic recording heads that have an overcoat of essentially amorphous carbon, as disclosed, for example, in U.S. Pat. No. 5,030,494 to Ahlert et al. and U.S. Pat. No. 5,075,287 to Doemer et al., both assigned to IBM Corporation. As described in the aforementioned patents, rotating rigid disk drives may include read/write transducers (or “heads”) supported on a carrier (or “slider”) that ride on a cushion or bearing of air above the surface of a magnetic recording disk when the disk is rotating at operating speed. The slider has an air-bearing surface (“ABS”), typically in the form of a plurality of rails, and is connected to a linear or rotary actuator by means of a suspension. There may be a stack of disks in the disk drive with the actuator supporting a number of sliders. The actuator moves the sliders radially so that each head may access the recording area of its associated disk surface. The slider in the disk drive is biased toward the disk surface by a small force from the suspension. The ABS of the slider is thus in contact with the disk surface from the time the disk drive is turned on until the disk reaches a speed sufficient to cause the slider to ride on the air bearing. The ABS of the slider is again in contact with the disk surface when the disk drive is turned off and the rotational speed of the disk fails below that necessary to create the air bearing. This type of disk drive is called a contact start/stop (CSS) disk drive. To provide wear resistance for the ABS in a CSS disk drive, a protective carbon overcoat may be placed on the slider rails. U.S. Pat. No. 5,159,508 describes a slider with air-bearing rails having an amorphous carbon overcoat that is adhered to the rails by a silicon adhesion layer.
The magnetic recording disk in a CSS rigid disk drive is typically a thin film disk comprising a substrate, such as a disk blank made of glass, ceramic, glassy carbon or an aluminum-magnesium (AlMg) alloy with a nickel-phosphorous (NiP) surface coating, and a cobalt-based magnetic alloy film formed by sputter deposition over the substrate. A protective overcoat, such as a sputter-deposited amorphous carbon film, is formed over the magnetic layer to provide corrosion resistance and wear resistance from the ABS of the slider. The overcoat may further include relatively small amounts of embedded iron (Fe), tungsten (W) or tungsten carbide (WC) to improve wear resistance and minimize the likelihood of damage to disk file components (see U.S. Pat. No. 5,030,494 to Ahlert et al., cited above). Such overcoats are typically formed by sputter deposition from a graphite target, and are generally called protective carbon overcoats, “diamondlike” carbon overcoats, amorphous carbon overcoats, or, in the case of those overcoats formed by sputter deposition in the presence of a hydrogen-containing gas, hydrogenated carbon overcoats. In addition to the magnetic layer and the protective overcoat, the thin film disk may also include a sputter-deposited underlayer, such as a layer of chromium (Cr) or a chromium-vanadium (CrV) alloy, between the substrate and the magnetic layer, and a sputter-deposited adhesion layer, such as a Cr, tungsten (W) or titanium (Ti) layer, between the magnetic layer and the protective overcoat.
A lubricant topcoat is conventionally applied as a uniform layer over the protective overcoat to prevent wear between the disk and head interface during drive operation. Excessive wear of the protective overcoat increases friction between the head and disk, thereby causing catastrophic drive failure. On the other hand, excess lubricant at the head-disk interface may cause high resistance between the head and disk (i.e., a high coefficient of friction between the head and disk at rest) such that, if the resistance is excessive, the drive cannot start and catastrophic failure may occur.
Due to the demand for increased recording density and faster data transfer rates, leading in part to the development of smoother disk surfaces and lower head flying heights, a continuing need exists for the development of new lubricants to serve as a lubricating topcoat overlying the protective overcoat. Such lubricants must perform a variety of different functions requiring diverse characteristics and attributes. For example, the lubricant forming the topcoat is preferably chemically inert, possesses a low vapor pressure, low surface tension, high thermal stability, stability under high shear stress, good boundary lubrication properties, and good adhesion to the underlying surface over the lifetime of the magnetic recording media. It is also important that the lubricant possesses the ability to maintain a uniform thickness on the magnetic recording media.
One of the most widely used lubricants is Fomblin Z-Tetraol™, a tetra-hydroxyl functionalized PFPE (Solvay Solexis, Inc.). Unfortunately, PFPEs tend to have poor solubility in relatively non-polar fluorinated solvents, which are commonly used as the lube bath solvents for deposition of functionalized PFPEs on disks for HDD applications. Even at low concentrations, a Fomblin Z-Tetraol™ solution in a hydrofluorocarbon or hydrofluoroether solvent, is found to be cloudy and turbid due to the poor solubility of the PFPE lubricant in the fluorinated solvents. Lubricant layers formed from Z-Tetraol™ also suffer from other disadvantages including relatively low durability and inadequate head clearance on smooth disk surfaces compared with other lubricants.
Other commercially available PFPE lubricants useful for hard disk drive lubricant layers include the A20H™ family of lubricants from Moresco (Matsumura Oil Research Corporation), which are generally described as perfluoropolyethers containing a cyclotriphosphazene ring. Two of such lubricants are designated as A20H-2000™ and A20H-4000™, having perfluoropolyether segment number average molecular weights of 2000 and 4000, respectively. While the A20H™ lubricants provide good slider-disk mechanical clearance in low-flying hard disk drives (<10 nm flying height) when employed as a neat lubricant on a rigid magnetic disk, they too suffer from several disadvantages. One such problem occurs due to the interactions between the head and the disk that leads to the formation of non-uniform lubricant distributions on the disk surface, variously known in the art as “lube moguls” and/or “lube ripples.”
In order to overcome some of the deficiencies of lubricants for hard disk drives, compositions of more than one lubricant have been described and provided in the art. For example, in U.S. Pat. No. 6,468,947 to Falcone et al., a lubricant composition is described comprising a first fluorpolyether and a second fluoropolyether having nitrogen containing end groups in which the composition is said to exhibit improved resistance to acid and thermal decomposition. U.S. Pat. No. 6,686,019 to Liu et al., describes a method for in situ stabilizing a composite lubricant of at least two mutually immiscible lubricant components (e.g., a phosphazene derivative and a perfluoropolyether lubricant) by treating a lubricant layer formed therefrom with UV radiation. The composite lubricant layer is said to have reduced or substantially eliminated phase separation of the lubricant components.
U.S. Pat. No. 5,908,817 to Perettie et al., also describes a lubricant comprising a mixture of a perfluorpolyether and a cyclic phosphazene in which the static and dynamic coefficients of friction are said to be low and balling effects (i.e., phase separation leading to chemical nonuniformity of the lubricant on the disk) are minimized.
In U.S. Pat. No. 6,605,335 to Tani et al., a magnetic recording media lubricant composition is described in which the lubricant comprises a dicyclotriphosphazene-perfluoropolyether compound having a specific structure, and compositions of such compounds that may be mixed with other lubricants such as Fombrin Z-dol™.
Despite advances in the art, a continuing need exists for improved lubricants for magnetic recording media, however, particularly for hard disk drive applications. The present invention addresses such needs by providing an improved lubricant composition that, among other advantages, demonstrates improved lubricant uniformity on hard disk surfaces, in particular, substantially reduced lubricant thickness variations compared with other lubricants or lubricant compositions.